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Sony Ericsson P990i is a smartphone and the successor of Sony Ericsson P910. The phone uses the UIQ 3 software platform, which is based upon Symbian OS 9.1 [link]

Sony Ericsson P990i technical specifications (network related only):

• GSM 900 / 1800 / 1900
• UMTS 2100
• GPRS Class 10 (4+1/3+2 slots), 32 - 48 kbps
• 3G, 384 kbps
• Wi-Fi 802.11b

Steps to install:

• Login to your box via SSH as root

• Put the Sony Ericsson to any available USB port.

• On the handset, choose "Phone Mode".

• Check if CentOS recognize the modem.

  tail /var/log/messages
  

• You'll see something like these:

  Mar  8 10:51:37 centos1 kernel: usb 2-1: new full speed USB device using uhci_hcd and address 6
  Mar  8 10:51:37 centos1 kernel: usb 2-1: configuration #1 chosen from 1 choice
  Mar  8 10:51:38 centos1 kernel: cdc_acm 2-1:1.1: ttyACM0: USB ACM device
  Mar  8 10:51:38 centos1 kernel: cdc_acm 2-1:1.3: ttyACM1: USB ACM device
  Mar  8 10:51:38 centos1 kernel: cdc_acm 2-1:1.5: ttyACM2: USB ACM device
  Mar  8 10:51:38 centos1 kernel: usbcore: registered new driver cdc_acm
  Mar  8 10:51:38 centos1 kernel: drivers/usb/class/cdc-acm.c: v0.25:USB Abstract Control Model driver for USB modems and ISDN adapters
  

  
  Means CentOS recognized the P990i as a modem.

• Create the modem configuration in /etc/wvdial.conf

  wvdialconf /etc/wvdial.conf
  

  You'll see something like these:
  

  Scanning your serial ports for a modem.
  
  ttyS0<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
  ttyS0<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
  ttyS0<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.
  ttyS1<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
  ttyS1<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
  ttyS1<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.
  Port Scan<*1>: S2   S3
  WvModem<*1>: Cannot get information for serial port.
  ttyACM0<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
  ttyACM0<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
  ttyACM0<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.
  WvModem<*1>: Cannot get information for serial port.
  ttyACM1<*1>: ATQ0 V1 E1 -- OK
  ttyACM1<*1>: ATQ0 V1 E1 Z -- OK
  ttyACM1<*1>: ATQ0 V1 E1 S0=0 -- OK
  ttyACM1<*1>: ATQ0 V1 E1 S0=0 &C1 -- OK
  ttyACM1<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 -- OK
  ttyACM1<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0 -- OK
  ttyACM1<*1>: Modem Identifier: ATI -- Sony Ericsson P990i
  ttyACM1<*1>: Speed 4800: AT -- OK
  ttyACM1<*1>: Speed 9600: AT -- OK
  ttyACM1<*1>: Speed 19200: AT -- OK
  ttyACM1<*1>: Speed 38400: AT -- OK
  ttyACM1<*1>: Speed 57600: AT -- OK
  ttyACM1<*1>: Speed 115200: AT -- OK
  ttyACM1<*1>: Speed 230400: AT -- OK
  ttyACM1<*1>: Speed 460800: AT -- OK
  ttyACM1<*1>: Max speed is 460800; that should be safe.
  ttyACM1<*1>: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0 -- OK
  WvModem<*1>: Cannot get information for serial port.
  ttyACM2<*1>: ATQ0 V1 E1 -- failed with 2400 baud, next try: 9600 baud
  ttyACM2<*1>: ATQ0 V1 E1 -- failed with 9600 baud, next try: 115200 baud
  ttyACM2<*1>: ATQ0 V1 E1 -- and failed too at 115200, giving up.
  
  Found an USB modem on /dev/ttyACM1.
  Modem configuration written to /etc/wvdial.conf.
  ttyACM1<Info>: Speed 460800; init "ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0"
  

• Done.

Good luck!

Update:
To use P990i with Indosat Broadband 3G see the following:
How to configure Sony Ericsson P990i as USB modem with Indosat Broadband 3G